I have had my Sigma 18-50 2.8 EX Macro HSM lens for over a year with no issues. Its mounted on my Nikon D90. When i put it on the the other day I got an F-- error. I fooled around with it a bit and found when I turn it back slightly (clockwise) the message goes away and I can shoot fine again. It seems though when you use the zoom sometimes it turns the lens just enough you get the F-- again. I thought this may be the D90 body but have no issues with other lenses. I mounted the Sigma on my old D50 and get the same error code.

Any idea what the issue is and how to fix it? Or will I need to return this to Sigma for repair?

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First, remove the lens and examine the contacts carefully with a magnifying glass. The contacts are spring loaded (for lack of a better term) so they keep in physical contact with the corresponding ones in the camera mount. Look to see that they are uniform and none of them are distorted in shape or smashed down. Also, check the finish for particles of dust or dirt that may be interfering, or for oxidation that may have formed. Something simple as a light brushing might clear it. A cotton swab moistened with alcohol maybe. If the metal is oxidized, a light burnishing with a clean pencil eraser will remove it. Exercise great care that you do nothing that will put contaminants into the lens. Your rear cap should be in place when the lens is removed from the camera.

While you are looking at the lens base it would also be a good time to check for any other irregularities like loose screws and other attachments.

Steve

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